renderers/data_source: fix resolve_file_url return value (fixes #5092)

URL resolvers must return a stream and an error code.

src/cmsplugin_blurp/renderers/data_source.py

@@ -191,7 +191,7 @@ class Data(object):
     def resolve_file_url(self):
         path = self.url[7:]
         try:
-            return file(path)
+            return file(path), None
         except Exception:
             log.exception('unable to resolve file URL: %r', self.url)